Characterization of Gravitational Microlensing Planetary Host Stars

The gravitational microlensing light curves that reveal the presence ofextrasolar planets generally yield the planet-star mass ratio and separation inunits of the Einstein ring radius. The microlensing method does not require thedetection of light from the planetary host star. This allows the detection ofplanets orbiting very faint stars, but it also makes it difficult to convertthe planet-star mass ratio to a value for the planet mass. We show that in manycases, the lens stars are readily detectable with high resolution space-basedfollow-up observations in a single passband. When the lens star is detected,the lens-source relative proper motion can also be measured, and this allowsthe masses of the planet and its host star to be determined and the star-planetseparation can be converted to physical units. Observations in multiplepassbands provide redundant information, which can be used to confirm thisinterpretation. For the recently detected super-Earth planet,OGLE-2005-BLG-169Lb, we show that the lens star will definitely be detectablewith observations by the Hubble Space Telescope (HST) unless it is a stellarremnant. Finally, we show that most planets detected by a space-basedmicrolensing survey are likely to orbit host stars that will be detected andcharacterized by the same survey.Comment: accepted for publication in ApJ, May 10, 200